This invention relates to catalysts for the preparation of polyethylene having a broad molecular weight distribution.
It is well known that linear polyethylene may be prepared by the polymerization of ethylene (optionally with one or more olefins or diolefins such as butene, hexene, octene or hexadiene) using a xe2x80x9cZieglerxe2x80x9d catalyst system which comprises a transition metal compound (such as a titanium halide or a vanadium halide and an aluminum alkyl). Polyethylene produced in this manner, particularly xe2x80x9clinear low density polyethylenexe2x80x9d, is in widespread commercial use. However, the conventional linear low density polyethylene (xe2x80x9clldpexe2x80x9d) made with Ziegler catalysts suffers from a number of deficiencies. Most notably, conventional lldpe is a heterogeneous product which contains a small fraction of low molecular weight wax and a comparatively large amount of very high molecular weight homopolymer. The heterogeneous nature of these polymers generally detracts from the physical properties made from them.
Accordingly, a great deal of effort has been directed towards the preparation of xe2x80x9chomogeneousxe2x80x9d lldpe resins which mitigate this problem. In particular, it is now well known to those skilled in the art that so-called xe2x80x9cmetallocenexe2x80x9d catalysts may be used to produce homogeneous lldpe resin. These homogeneous resins are, however, not without problems. Most notably, these homogeneous resins typically have a narrow molecular weight distribution and are difficult to xe2x80x9cprocessxe2x80x9d or convert into finished polyethylene products. Thus, efforts to improve the processability of homogeneous polyethylene resin by broadening the molecular weight distribution have been made and are disclosed, for example in U.S. Pat. Nos. 4,530,914; 4,701,432; 4,935,474; 4,937,299; 5,124,418 and 5,183,867.
In copending and commonly assigned patent applications there are disclosed certain phosphinimine catalysts which may be used to produce homogeneous polyethylene.
The present invention provides a catalyst system for the (co)polymerization of ethylene to polyethylene having a broad molecular weight distribution, said catalyst system comprising:
a) at least two different mono or di-phosphinimine catalysts;
b) at least one cocatalyst; and
c) a particulate support.
It will be understood by those skilled in the art that said two different mono or di-phosphinimine catalysts must have different propagation and/or termination constants in order to produce a polymer having a broad molecular weight distribution.
As used herein, the term xe2x80x9cmono-phosphinimine catalystxe2x80x9d refers to a catalyst having a single phosphinimine ligand and xe2x80x9cdi-phosphinimine catalystxe2x80x9d refers to a catalyst having two phosphinimine ligands.
It is required that at least two different catalysts be employed. The differences may be achieved, for example, by the use of different transition metals, different cyclopentadienyl ligands, different phosphinimine ligands or combinations thereof.
It is preferred that each of at least two phosphinimine catalysts used in this invention is defined by the formula:
(Cp)mM(Pl)n(L)q
wherein Pl is a phosphinimine ligand (see section 1.1 below); Cp is a cyclopentadienyl-type ligand (section 1.2 below); L is a monoanionic activatable ligand (section 1.3 below); m is a metal selected from Ti, Hf and Zr; and
wherein m is 0 or 1; n is 1 or 2; and m+n+q=the valence of the metal m.
The two phosphinimine catalysts must be different as further described in the Examples.
The most preferred catalysts are those in which the metal is 4 valent. For example, a catalyst may be a cyclopentadienyl (phosphinimine) complex of titanium, zirconium, or hafnium having two additional, monoanionic ligands. It is particularly preferred that each catalyst contains one phosphinimine ligand, one cyclopentadienyl ligand and two chloride or alkyl ligands.
1.1 Phosphinimine Ligand
Each catalyst must contain at least one phosphinimine ligand which is covalently bonded to the metal. Phosphinimine ligands are defined by the formula: 
wherein each R1 is independently selected from the group consisting of a hydrogen atom, a halogen atom, C1-20 hydrocarbyl radicals which are unsubstituted by or further substituted by a halogen atom, a C1-8 alkoxy radical, a C6-10 aryl or aryloxy radical, an amido radical, a silyl radical of the formula:
xe2x80x94Sixe2x80x94(R2)3
wherein each R2 is independently selected from the group consisting of hydrogen, a C1-8 alkyl or alkoxy radical, C6-10 aryl or aryloxy radicals, and a germanyl radical of the formula:
Gexe2x80x94(R2)3
wherein R2 is as defined above.
The preferred phosphinimines are those in which each R1 is a hydrocarbyl radical. A particularly preferred phosphinimine is tri-(tertiary butyl) phosphinimine (i.e. where each R1 is a tertiary butyl group).
1.2 Cyclopentadienyl Ligands
As used herein, the term cyclopentadienyl-type ligand is meant to convey its conventional meaning, namely a ligand having a five carbon ring which is bonded to the metal via eta-5 bonding. Thus, the term xe2x80x9ccyclopentadienyl-typexe2x80x9d includes unsubstituted cyclopentadienyl, substituted cyclopentadienyl, unsubstituted indenyl, substituted indenyl, unsubstituted fluorenyl and substituted fluorenyl. An exemplary list of substituents for a cyclopentadienyl ligand includes the group consisting of C1-10 hydrocarbyl radical (which hydrocarbyl substituents are unsubstituted or further substituted); a halogen atom, C1-8 alkoxy radical, a C6-10 aryl or aryloxy radical; an amido radical which is unsubstituted or substituted by up to two C1-8 alkyl radicals; a phosphido radical which is unsubstituted or substituted by up to two C1-8 alkyl radicals; silyl radicals of the formula xe2x80x94Sixe2x80x94(R)3 wherein each R is independently selected from the group consisting of hydrogen, a C1-8 alkyl or alkoxy radical C6-10 aryl or aryloxy radicals; germanyl radicals of the formula Gexe2x80x94(R)3 wherein R is as defined directly above.
1.3 Activatable Ligand
The term xe2x80x9cactivatable ligandxe2x80x9d refers to a ligand which may be activated by a cocatalyst, (or xe2x80x9cactivatorxe2x80x9d), to facilitate olefin polymerization. Exemplary activatable ligands are independently selected from the group consisting of a hydrogen atom, a halogen atom, a C1-10 hydrocarbyl radical, a C1-10 alkoxy radical, a C5-10 aryl oxide radical; each of which said hydrocarbyl, alkoxy, and aryl oxide radicals may be unsubstituted by or further substituted by a halogen atom, a C1-8 alkyl radical, a C1-8 alkoxy radical, a C6-10 aryl or aryloxy radical, an amido radical which is unsubstituted or substituted by up to two C1-8 alkyl radicals; a phosphido radical which is unsubstituted or substituted by up to two C1-8 alkyl radicals.
The number of activatable ligands depends upon the valency of the metal and the valency of the activatable ligand. The preferred first catalyst metals are group 4 metals in their highest oxidation state (i.e. 4+) and the preferred activatable ligands are monoanionic (such as a halidexe2x80x94especially chloride or an alkylxe2x80x94especially methyl). Thus, the preferred first catalyst contains a phosphinimine ligand, a cyclopentadienyl ligand and two chloride (or methyl) ligands bonded to the group 4 metal. In some instances, the metal of the first catalyst component may not be in the highest oxidation state. For example, a titanium (III) component would contain only one activatable ligand. Also, it is permitted to use a dianionic activatable ligand although this is not preferred.
2. Description of Cocatalyst
The catalyst components described in part 1 above are used in combination with at least one cocatalyst (or xe2x80x9cactivatorxe2x80x9d) to form an active catalyst system for olefin polymerization as described in more detail in sections 2.1 and 2.2 below.
2.1 Alumoxanes
The alumoxane may be of the formula:
(R4)2AlO(R4AlO)mAl(R4)2
wherein each R4 is independently selected from the group consisting of C1-20 hydrocarbyl radicals and m is from 0 to 50, preferably R4 is a C1-4 alkyl radical and m is from 5 to 30. Methylalumoxane (or xe2x80x9cMAOxe2x80x9d) in which each R is methyl is the preferred alumoxane.
Alumoxanes are well known as cocatalysts, particularly for metallocene-type catalysts. Alumoxanes are also readily available articles of commerce.
The use of an alumoxane cocatalyst generally requires a molar ratio of aluminum to the transition metal in the catalyst from 20:1 to 1000:1. Preferred ratios are from 50:1 to 250:1.
2.2 xe2x80x9cIonic Activatorsxe2x80x9d Cocatalysts
So-called xe2x80x9cionic activatorsxe2x80x9d are also well known for metallocene catalysts. See, for example, U.S. Pat. No. 5,198,401 (Hlatky and Turner) and U.S. Pat. No. 5,132,380 (Stevens and Neithamer).
Whilst not wishing to be bound by any theory, it is thought by those skilled in the art that xe2x80x9cionic activatorsxe2x80x9d initially cause the abstraction of one or more of the activatable ligands in a manner which ionizes the catalyst into a cation, then provides a bulky, labile, non-coordinating anion which stabilizes the catalyst in a cationic form. The bulky, non-coordinating anion permits olefin polymerization to proceed at the cationic catalyst center (presumably because the non-coordinating anion is sufficiently labile to be displaced by monomer which coordinates to the catalyst). Preferred ionic activators are boron-containing ionic activators described in (i)xe2x80x94(iii) below:
(i) compounds of the formula [R5]+ [B(R7)4]xe2x88x92 wherein B is a boron atom, R5 is an aromatic hydrocarbyl (e.g. triphenyl methyl cation) and each R7 is independently selected from the group consisting of phenyl radicals which are unsubstituted or substituted with from 3 to 5 substituents selected from the group consisting of a fluorine atom, a C1-4 alkyl or alkoxy radical which is unsubstituted or substituted by a fluorine atom; and a silyl radical of the formula xe2x80x94Sixe2x80x94(R9)3; wherein each R9 is independently selected from the group consisting of a hydrogen atom and a C1-4 alkyl radical; and
(ii) compounds of the formula [(R8)tZH]+[B(R7)4]xe2x88x92 wherein B is a boron atom, H is a hydrogen atom, Z is a nitrogen atom or phosphorus atom, t is 2 or 3 and R8 is selected from the group consisting of C1-8 alkyl radicals, a phenyl radical which is unsubstituted or substituted by up to three C1-4 alkyl radicals, or one R8 taken together with the nitrogen atom may form an anilinium radical and R7 is as defined above; and
(iii) compounds of the formula B(R7)3 wherein R7 is as defined above.
In the above compounds preferably R7 is a pentafluorophenyl radical, and R5 is a triphenylmethyl cation, Z is a nitrogen atom and R8 is a C1-4 alkyl radical or R8 taken together with the nitrogen atom forms an anilinium radical which is substituted by two C1-4 alkyl radicals.
The xe2x80x9cionic activatorxe2x80x9d may abstract one or more activatable ligands so as to ionize the catalyst center into a cation but not to covalently bond with the catalyst and to provide sufficient distance between the catalyst and the ionizing activator to permit a polymerizable olefin to enter the resulting active site.
Examples of ionic activators include:
triethylammonium tetra(phenyl)boron,
tripropylammonium tetra(phenyl)boron,
tri(n-butyl)ammonium tetra(phenyl)boron,
trimethylammonium tetra(p-tolyl)boron,
trimethylammonium tetra(o-tolyl)boron,
tributylammonium tetra(pentafluorophenyl)boron,
tripropylammonium tetra(o,p-dimethylphenyl)boron,
tributylammonium tetra(m,m-dimethylphenyl)boron,
tributylammonium tetra(p-trifluoromethylphenyl)boron,
tributylammonium tetra(pentafluorophenyl)boron,
tri(n-butyl)ammonium tetra(o-tolyl)boron,
N,N-dimethylanilinium tetra(phenyl)boron,
N,N-diethylanilinium tetra(phenyl)boron,
N,N-diethylanilinium tetra(phenyl)n-butylboron,
N,N-2,4,6-pentamethylanilinium tetra(phenyl)boron,
di-(isopropyl)ammonium tetra(pentafluorophenyl)boron,
dicyclohexylammonium tetra(phenyl)boron,
triphenylphosphonium tetra(phenyl)boron,
tri(methylphenyl)phosphonium tetra(phenyl)boron,
tri(dimethylphenyl)phosphonium tetra(phenyl)boron,
tropilliur tetrakispentafluorophenyl borate,
triphenylmethylium tetrakispentafluorophenyl borate,
benzene(diazonium)tetrakispentafluorophenyl borate,
tropillium phenyltrispentafluorophenyl borate,
triphenylmethylium phenyltrispentafluorophenyl borate,
benzene(diazonium)phenyltrispentafluorophenyl borate,
tropillium tetrakis(2,3,5,6-tetrafluorophenyl)borate,
triphenylmethylium tetrakis(2,3,5,6-tetrafluorophenyl)borate,
benzene(diazonium)tetrakis(3,4,5-trifluorophenyl)borate,
tropillium tetrakis(3,4,5-trifluorophenyl)borate,
benzene(diazonium)tetrakis(3,4,5-trifluorophenyl)borate,
tropillium tetrakis(1,2,2-trifluoroethenyl)borate,
triphenylmethylium tetrakis(1,2,2-trifluoroethenyl)borate,
benzene(diazonium)tetrakis(1,2,2-trifluoroethenyl)borate,
tropillium tetrakis(2,3,4,5-tetrafluorophenyl)borate,
triphenylmethylium tetrakis(2,3,4,5-tetrafluorophenyl)borate, and
benzene(diazonium)tetrakis(2,3,4,5-tetrafluorophenyl)borate.
Readily commercially available ionic activators include: N,N-dimethylaniliniumtetrakispentafluorophenyl borate, triphenylmethylium tetrakispentafluorophenyl borate, and trispentafluorophenyl borane.
Heterogeneous Catalysts
The catalysts of this invention are used in a particulate or heterogeneous form. It is also highly preferred that the catalyst does not cause reactor fouling. The art of preparing heterogeneous catalysts which do not lead to reactor fouling is not adequately understood, though it is generally accepted that the catalytic material should be very well anchored to the support so as to reduce the incidence of fouling resulting from the deposition of catalyst or cocatalyst which has dissociated from the support.
In general, heterogeneous catalysts may be grouped into three main categories:
3.1. xe2x80x9cUnsupportedxe2x80x9d Alumoxane/Catalyst Mixtures
These catalysts may be easily prepared by evaporating the solvent or diluent from a liquid mixture of an alumoxane and the two catalyst components. The resulting product is a solid at room temperature due to the comparatively high molecular weight of the alumoxane. Thus, the alumoxane forms the support. This may be done xe2x80x9cin-situxe2x80x9d (i.e. in the reactor) by spraying the alumoxane and catalysts into the reactor.
There are two disadvantages to using alumoxane as the support. The first is cost: alumoxanes are comparatively expensivexe2x80x94and the alumoxane is used as an expensive xe2x80x9csupportxe2x80x9d material). The second is xe2x80x9creaction continuity/foulingxe2x80x9d (i.e. the alumoxane may partially melt under polymerization conditions, leading to reactor instability/fouling).
3.2. Conventional Supported Catalysts
Supported catalysts are conventionally prepared by depositing the catalyst components and a cocatalyst on a very porous metal oxide support such as silica or alumina. The catalyst and cocatalyst are substantially contained within the pore structure of the metal oxide particle. This means that a comparatively large metal oxide particle is used (typically particle size of from 40 to 80 microns). The preparation of this type of supported catalyst is described in U.S. Pat. No. 4,808,561 (Welborn, to Exxon).
Metal oxides, especially silica or alumina, are preferred support materials. Other supports known to those skilled in the art include polymers (such as polyolefins or polystyrene-co-divinyl benzene), zeolites and ceramics.
3.3. Filled/Spray Dried Catalysts
This method of catalyst preparation is also well known. For example, U.S. Pat. Nos. 5,648,310; 5,674,795 and 5,672,669 (all to Union Carbide) teach the preparation of a heterogeneous catalyst by spray drying a mixture which contains a metallocene catalyst, an alumoxane cocatalyst and a xe2x80x9cfillerxe2x80x9d which is characterized by having a very small particle size (less than one micron) and by being unreactive with the catalyst and cocatalyst. The examples illustrate the use of very fine particle size xe2x80x9cfumedxe2x80x9d silica which has been treated to reduce the concentration of surface hydroxyls. The resulting catalysts exhibit good productivity. Moreover, they offer the potential to provide a catalyst which is not prone to xe2x80x9chot spotsxe2x80x9d (as the catalyst may be evenly distributed, at low concentration, throughout the heterogeneous matrix). However, these catalysts suffer from the potential disadvantage of being very friable because they are prepared with a fine, xe2x80x9cinertxe2x80x9d filler material which does not react with/anchor to the catalyst or cocatalyst.
Friable catalyst particles lead to the formation of xe2x80x9cfinesxe2x80x9d in the polyethylene product, and may also aggravate reactor fouling problems.
An alternative approach is the preparation of spray dried catalysts using a hydrotalcite as a xe2x80x9creactivexe2x80x9d filler (as opposed to the unreactive filler described in the above mentioned U.S. Pat. No. to Union Carbide). This method of catalyst preparation is described in more detail in a commonly assigned patent application. Either approach is suitable for use with the catalysts of this invention.
4. Polymerization Processes
The catalysts of this invention are suitable for use in any conventional olefin polymerization process, such as the so-called xe2x80x9cgas phasexe2x80x9d, xe2x80x9cslurryxe2x80x9d, xe2x80x9chigh pressurexe2x80x9d or xe2x80x9csolutionxe2x80x9d polymerization processes.
The polymerization process according to this invention uses ethylene and may include other monomers which are copolymerizable therewith such as other alpha olefins (having from three to ten carbon atoms, preferably butene, hexene or octene) and, under certain conditions, dienes such as hexadiene isomers, vinyl aromatic monomers such as styrene or cyclic olefin monomers such as norbomene.
The present invention may also be used to prepare elastomeric co- and terpolymers of ethylene, propylene and optionally one or more diene monomers. Generally, such elastomeric polymers will contain about 50 to abut 75 weight % ethylene, preferably about 50 to 60 weight % ethylene and correspondingly from 50 to 25% of propylene. A portion of the monomers, typically the propylene monomer, may be replaced by a conjugated diolefin. The diolefin may be present in amounts up to 10 weight % of the polymer although typically is present in amounts from about 3 to 5 weight %. The resulting polymer may have a composition comprising from 40 to 75 weight % of ethylene, from 50 to 15 weight % of propylene and up to 10 weight % of a diene monomer to provide 100 weight % of the polymer. Preferred but not limiting examples of the dienes are dicyclopentadiene, 1,4-hexadiene, 5-methylene-2-norbornene, 5-ethylidene-2-norbomene and 5-vinyl-2-norbornene. Particularly preferred dienes are 5-ethylidene-2-norbomene and 1,4-hexadiene.
The polyethylene polymers which may be prepared in accordance with the present invention typically comprise not less than 60, preferably not less than 70 weight % of ethylene and the balance one or more C4-10 alpha olefins, preferably selected from the group consisting of 1 -butene, 1-hexene and 1-octene. The polyethylene prepared in accordance with the present invention may be linear low density polyethylene having density from about 0.910 to 0.935 g/cc. The present invention might also be useful to prepare polyethylene having a density below 0.910 g/ccxe2x80x94the so-called very low and ultra low density polyethylenes.
The catalyst of this invention is preferably used in a slurry polymerization process or a gas phase polymerization process.
The typical slurry polymerization process uses total reactor pressures of up to about 50 bars and reactor temperature of up to about 200xc2x0 C. The process employs a liquid medium (e.g. an aromatic such as toluene or an alkane such as hexane, propane or isobutane) in which the polymerization takes place. This results in a suspension of solid polymer particles in the medium. Loop reactors are widely used in slurry processes. Detailed descriptions of slurry polymerization processes are widely reported in the open and patent literature.
In general, a fluidized bed gas phase polymerization reactor employs a xe2x80x9cbedxe2x80x9d of polymer and catalyst which is fluidized by a flow of monomer which is at least partially gaseous. Heat is generated by the enthalpy of polymerization of the monomer flowing through the bed. Unreacted monomer exits the fluidized bed and is contacted with a cooling system to remove this heat. The cooled monomer is then re-circulated through the polymerization zone together with xe2x80x9cmake-upxe2x80x9d monomer to replace that which was polymerized on the previous pass. As will be appreciated by those skilled in the art, the xe2x80x9cfluidizedxe2x80x9d nature of the polymerization bed helps to evenly distribute/mix the heat of reaction and thereby minimize the formation of localized temperature gradients (or xe2x80x9chot spotsxe2x80x9d). Nonetheless, it is essential that the heat of reaction be properly removed so as to avoid softening or melting of the polymer (and the resultant-and highly undesirablexe2x80x94xe2x80x9creactor chunksxe2x80x9d). The obvious way to maintain good mixing and cooling is to have a very high monomer flow through the bed. However, extremely high monomer flow causes undesirable polymer entrainment.
An alternative (and preferable) approach to high monomer flow is the use of an inert condensable fluid which will boil in the fluidized bed (when exposed to the enthalpy of polymerization), then exit the fluidized bed as a gas, then come into contact with a cooling element which condenses the inert fluid. The condensed, cooled fluid is then returned to the polymerization zone and the boiling/condensing cycle is repeated.
The above-described use of a condensable fluid additive in a gas phase polymerization is often referred to by those skilled in the art as xe2x80x9ccondensed mode operationxe2x80x9d and is described in additional detail in U.S. Pat. No. 4,543,399 and U.S. Pat. No. 5,352,749. As noted in the ""399 reference, it is permissible to use alkanes such as butane, pentanes or hexanes as the condensable fluid and the amount of such condensed fluid preferably does not exceed about 20 weight per cent of the gas phase.
Other reaction conditions for the polymerization of ethylene which are reported in the ""399 reference are:
Preferred Polymerization Temperatures: about 75xc2x0 C. to about 115xc2x0 C. (with the lower temperatures being preferred for lower melting copolymersxe2x80x94especially those having densities of less than 0.915 g/ccxe2x80x94and the higher temperatures being preferred for higher density copolymers and homopolymers); and
Pressure: up to about 1000 psi (with a preferred range of from about 100 to 350 psi for olefin polymerization).
The ""399 reference teaches that the fluidized bed process is well adapted for the preparation of polyethylene but further notes that other monomers may be employedxe2x80x94as is the case in the process of this invention.